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United States Patent |
5,050,946
|
Hathaway
,   et al.
|
September 24, 1991
|
Faceted light pipe
Abstract
A light pipe used for backlighting liquid crystal displays has a planar
front surface and a stairstepped or faceted back surface. Light is
injected from the ends of the light pipe from cold or hot cathode,
apertured, fluorescent lamps. The cold cathode lamps are preferably
insulated to raise their operating temperature. The back surface has a
series of planar portions parallel to the front surface connected by
facets, which are angled so that the injected light reflects off the
facets and through the front surface. A reflector having a planar, highly
reflective, highly scattering surface or a sawtoothed or grooved upper
surface is located adjacent to and parallel with the light pipe back
surface to reflect light escaping from the back surface back through the
light pipe to exit the front surface. The axis of grooves is preferably
slightly skewed from the facet axis to reduce moire pattern development. A
low scattering or loss diffuser is located adjacent to and parallel with
the light pipe front surface to reduce moire pattern development. The
liquid crystal display is located over the low scattering diffuser. A
separate injector may be located between the lamp and the light pipe to
better couple the light into the light pipe.
Inventors:
|
Hathaway; Kevin J. (San Jose, CA);
Knox, Jr.; Richard M. (Houston, TX);
Arego; Douglas A. (Spring, TX);
Kornfuerhrer; Gaylon R. (Cypress, TX)
|
Assignee:
|
Compaq Computer Corporation (Houston, TX)
|
Appl. No.:
|
589325 |
Filed:
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September 27, 1990 |
Current U.S. Class: |
385/33; 349/64; 349/65; 349/99; 362/27; 362/309; 362/341; 362/561; 362/614; 385/37; 385/146; 385/901 |
Intern'l Class: |
G02B 006/00 |
Field of Search: |
350/96.10,96.15,96.18,96.19
362/309,341,27,31,32
|
References Cited
U.S. Patent Documents
4257084 | Mar., 1981 | Reynolds | 362/31.
|
4277817 | Jul., 1981 | Hehr | 362/31.
|
4323951 | Apr., 1982 | Pasco | 362/27.
|
4528617 | Jul., 1985 | Blackington | 362/32.
|
4706173 | Nov., 1987 | Hamada et al. | 362/341.
|
4799137 | Jan., 1989 | Aho | 362/309.
|
4883333 | Nov., 1989 | Yanez | 350/96.
|
Foreign Patent Documents |
3825436 | Mar., 1989 | DE | 350/96.
|
0073206 | Apr., 1987 | JP | 350/96.
|
0271301 | Nov., 1988 | JP | 350/96.
|
0287803 | Nov., 1988 | JP | 350/96.
|
Primary Examiner: Epps; Georgia
Attorney, Agent or Firm: Pravel, Gambrell, Hewitt, Kimball & Krieger
Claims
We claim:
1. A system for backlighting a liquid crystal display, comprising:
a light pipe having a generally planar front surface for providing light to
the liquid crystal display, having a faceted back surface wherein said
back surface includes a plurality of generally planar portions parallel to
said front surface and a plurality of facets formed at an angle to said
front surface and located connecting said back surface parallel portions,
and having at least one end surface for receiving light to be transmitted
through said front surface;
light source means located adjacent each said end surface for receiving
light of said light pipe for providing light to said light pipe; and
reflector means located adjacent to and generally parallel to said light
pipe back surface for reflecting light back through said light pipe.
2. The system of claim 1, wherein said reflector means is generally planar
and has a front surface adjacent to said light pipe back surface, said
reflector means front surface including a series of grooves, the
longitudinal axis of said grooves extending somewhat parallel to the
longitudinal axis of said facets.
3. The system of claim 2, wherein the longitudinal axis of said grooves is
somewhat askew of the longitudinal axis of said facets.
4. The system of claim 1, wherein said reflector means is generally planar
and has a front surface adjacent to said light pipe back surface, said
reflector means front surface being highly reflective and highly
scattering.
5. The system of claims 3 or 4, further comprising:
injector means between said light source means and said light pipe for
coupling light produced by said light source means to said light pipe.
6. The system of claim 5, wherein said injector means has a flat surface
facing said light source means.
7. The system of claim 6, wherein said injector means flat surface is
coated with an anti-reflective coating.
8. The system of claim 5, wherein said injector means includes index
matching material located between and contacting said light source means
and said light pipe.
9. The system of claim 5, wherein said injector means is shaped to
generally conform to the surface of said light source means.
10. The system of claim 5, wherein said injector means includes a surface
having a fresnel lens developed thereon.
11. The system of claim 10, wherein said fresnel lens is a cylindrical
fresnel lens.
12. The system of claims 3 or 4, wherein each said end for receiving light
of said light pipe has a flat surface.
13. The system of claim 12, wherein said end is coated with an
anti-reflective coating.
14. The system of claim 12 wherein said end has a fresnel lens developed
thereon.
15. The system of claim 14, wherein said fresnel lens is a cylindrical
lens.
16. The system of claims 3 or 4, wherein each said end for receiving light
of said light pipe is shaped to generally conform to the surface of said
light source means.
17. The system of claims 3 or 4, wherein said light source means includes
fluorescent lamps.
18. The system of claim 17, wherein said lamps are reflector lamps.
19. The system of claim 17, wherein said lamps are aperture lamps.
20. The system of claim 17, wherein said lamps are cold cathode lamps.
21. The system of claim 20, wherein said lamps are partially encompassed by
insulation.
22. The system of claim 21, wherein said insulation includes a reflective
surface facing said lamps.
23. The system of claim 17, wherein said lamps are hot cathode lamps.
24. The system of claims 3 or 4, wherein said light source means includes
uniform dispersion fluorescent lamps.
25. The system of claim 24, wherein said light means further includes
reflectors formed around said lamps to reflect light to said light pipe.
26. The system of claims 3 or 4, wherein said light pipe is formed of
polymethyl methacrylate.
27. The system of claims 3 or 4, wherein the angle of said facets from said
parallel portion is between 90 and 180 degrees.
28. The system of claim 27, wherein the angle is approximately 135 degrees.
29. The system of claims 3 or 4, wherein the pitch defined by the distance
from successive facets is less than that required to exceed the visual
threshold of a human being.
30. The system of claim 29, wherein said pitch is randomly varied.
31. The system of claim 29, wherein said pitch is uniformly varied to a
maximum of approximately 1000 per inch.
32. The system of claims 3 or 4, wherein the facet height between
successive parallel portions is varied between two limits.
33. The system of claim 32, wherein said facet height limits are
approximately 1 and 100 microns.
34. The system of claims 3 or 4, further comprising a diffuser located
adjacent to and generally parallel to said light pipe front surface.
35. The system of claim 1, further comprising a low scattering diffuser
located adjacent to and generally parallel to said light pipe front
surface.
36. The system of claims 3 or 4, wherein said facets are generally planar.
37. The system of claims 3 or 4, wherein said facets are portions of a
generally cylindrical surface.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to backlighting systems used with liquid crystal
displays, and more particularly to light pipe systems.
2. Description of the Related Art
Liquid crystal displays (LCD's) are commonly used in portable computer
systems, televisions and other electronic devices. An LCD requires a
source of light for operation because the LCD is effectively a light
valve, allowing transmission of light in one state and blocking
transmission of light in a second state. Backlighting the LCD has become
the most popular source of light in personal computer systems because of
the improved contrast ratios and brightnesses possible. Because
conventional monochrome LCD's are only approximately 12% transmissive and
color LCD's are only approximately 2% transmissive, relative large amounts
of uniform light are necessary to provide a visible display. If power
consumption and space were not of concern the necessary level and
uniformity of backlight could be obtained.
However, in portable devices power consumption, which directly effects
battery life, and space are major concerns. Thus there is a need to obtain
a sufficiently uniform and bright backlight level with as little power as
possible in as little space as possible at, of course, as low a cost as
possible.
Numerous designs exist which trade off various of these goals to achieve a
balanced display. Several of these designs, such as light curtains and
light pipes, are shown in the figures and will be described in detail
later. The designs generally trade off uniformity of backlighting for
space or efficiency. The designs utilize various scattering means and a
final diffuser before the light is presented to the LCD. The scattering
means and the diffusers both allow loss of light and thus reduce the
efficiency of the transfer from the light source to the LCD. While the
designs are adequate in some cases, the demands for longer battery life
with monochrome LCD's or equal battery life with color LCD's are present,
as is a desire for the use of less space.
SUMMARY OF THE INVENTION
The present invention is a faceted, parallel surface light pipe design.
Light sources, preferably reflector or apertured fluorescent lamps, but
alternatively uniform lamps, supply light to one or both ends of a light
pipe. The front surface of the light pipe, on which is positioned a low
loss diffuser, which in turn is in contact with the LCD, is planar, while
the back surface of the light pipe is generally parallel to the front
surface, but has a stair stepped or faceted surface. The facets are
preferably formed at an angle so that the light injected into the ends of
the light pipe is reflected off the facets and through the front surface.
The pitch or step length of the facets is such that the faceting structure
is not visible to the human eye. The step height of the facets is
preferably in the micron range and may increase with the distance from the
lamp. A planar, white, diffuse reflector, which is highly reflective and
high scattering, is positioned parallel to the back surface of the
lightpipe. This allows light leaving the back surface to be reflected back
through the front surface of the light pipe. Alternatively, the reflector
can have a sawtoothed or grooved surface. The axis of the sawtooth ridges
is preferably slightly askew the axis of the facets to reduce the effects
of moire pattern development. The reflections can be satisfactorily
controlled so that little light is returned to the light source, little
light leaves the other end of the light pipe and little light is trapped
in the light pipe.
This design is in contrast to the low efficiency of the various scattering
techniques of the prior art which allow the losses described. The pitch
and step height are sufficient so that a conventional diffuser is not
required before the LCD, thus allowing further relative increased light
transmission and efficiency. However, a low loss diffuser is preferably
located between the light pipe and the display to overcome moire pattern
development. Various designs of the end of the light pipe and the actual
facet profile and pitch can be used to alter specific aspects of the
transmission to vary the light output.
BRIEF DESCRIPTION OF THE DRAWINGS
A better understanding of the prior art and the present invention can be
obtained when the following detailed description of the preferred
embodiment is considered in conjunction with the following drawings, in
which:
FIGS. 1-4 are views of various backlighting systems of the prior art;
FIG. 5 is a view of a backlighting system according to the present
invention including a light pipe and light sources;
FIGS. 6 and 7 are greatly enlarged views of portions of the backlighting
system of FIG. 5;
FIGS. 8, 9A, 9B and 10 are greatly enlarged views of portions of the light
pipe of FIG. 5 showing light action;
FIG. 11 is a greatly enlarged view of an alternate injector according to
the present invention;
FIG. 12 is a greatly enlarged, view of a facet of the light pipe of FIG. 5;
FIG. 13 is an alternate single source backlighting system according to the
present invention; and
FIGS. 14 to 17 are alternative designs for a lamp reflector according to
the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Prior to discussing the present invention, it is considered appropriate to
further discuss various designs in the prior art to explain the present
technology and thus make clear the scope of the present invention.
FIG. 1 generally discloses a conventional light curtain system used in
providing backlight to an LCD. Two uniform output cold cathode florescent
lamps 20 and 22 are the basic light source for the system S1. A reflector
24 generally having a white reflective surface facing the lamps 20 and 22
is used to redirect the light being emitted by the lamps 20 and 22 in
directions other than towards the LCD D. A light blocking layer 26 is used
to reduce any hot, nonuniform spots which would occur directly over the
lamps 20 and 22 to provide a first level of uniformity to the light. The
blocking layer 26 is preferably formed of a variable opacity mylar
material, with the material being very opaque near the lamps 20 and 22 and
becoming more translucent or transparent away from the lamps. This
variable opacity is generally provided by a printed pattern on the surface
of the blocking layer 26. However, because the light is not sufficiently
uniform after passing through the blocking layer 26, a diffuser 28, which
is generally a translucent plastic material, is used to further diffuse
the light and produce a more uniform display. However, the diffuser
generally reduces the light transmission by approximately 10% to 50%,
which greatly reduces the efficiency of the overall backlighting system
S1. The light curtain system S1 is relatively thick and as the lamps are
placed closer to the blocking layer alignment problems increase, reducing
the capability to economically manufacture the system S1.
Two variations of similar light pipe systems are shown in FIGS. 2 and 3 and
are generally referred to as systems S2 and S3. Both systems again
generally use uniform emission lamps 20 and 22, but the lamps are located
at the ends of a light pipe 30. White reflectors 32 and 34 are provided
around the lamps 20 and 22 so that the uniform light is directed into the
light pipe 30. The light pipe 30 includes a variable density scattering
structure so that the light is projected out the front surface 36 of the
light pipe 30, through the diffuser 28 and through the LCD D. In the
backlighting system S2 the light pipe 30 uses titanium oxide particles or
other particles located in the light pipe 30 to perform the scattering
function. Preferably the density of the particles is greater near the
center of the display and lesser near the ends of the display near the
lamps 20 and 22 to produce a uniform light because of the effective light
density, which reduces approaching the center of the light pipe 30. A
mirrored or fully reflective surface 38 is applied to the back surface 37
of the light pipe 30 so that any light which is scattered in that
direction is reflected in an attempt to have the light transmitted through
the front surface 36 of the light pipe 30. However, this light might again
be scattered and so various losses can occur. The backlighting system S3
uses a scattering structure printed on the front surface 42 of the light
pipe 40 to provide the scattering effect. In both systems S2 and S3 a
diffuser 28 is required to provide a sufficiently uniform light source to
the LCD D. In these designs light can become trapped in the light pipe 40
and can readily be transmitted from one end to the other and thus be lost,
reducing overall efficiency.
An alternate prior art light pipe design is shown in FIG. 4, and is
generally referred to by S4. In this case a double quadratic wedge light
pipe 44 is used in contrast to the parallel light pipes 30 and 40 of the
systems S2 and S3. The back surface 46 of the light pipe 44 is a
relatively constant, diffuse surface, with the front surface 47 being a
clear or specular surface. The curve formed by the back surface 46 is a
quadratic curve such that more light which impinges on the back surfaces
is reflected through the front surface as the light approaches the center
of the light pipe 44. In this way a relatively uniform light source can be
developed, but a diffuser 28 is still required to provide an adequately
uniform source. This design has problems in that some light does leak out
at low angles out the back and in some cases light is sent back to the
source. Additionally, there are some problems at the exact center of the
display.
Thus while the light pipe designs S2, S3 and S4 are generally thinner
designs than the light curtain system S1, they have problems related to
having to turn the light generally ninety degrees and thus have a lower
efficiency than the light curtain design, which in turn has the drawback
it is a relatively thick design which limits the design possibilities in
portable computer systems and television applications.
A backlight system according to the present invention, generally referred
to as S5, is shown in FIG. 5. A faceted, dual source light pipe 100 is
coupled to an LCD D. FIG. 5 shows two alternate lamp variations. In one
variation a uniform dispersion lamp 102 may be located adjacent to an
optional separate injector 104. The lamp 102 is preferably surrounded by a
reflector 106. The separate injector 104 is used to couple the transmitted
light from the lamp 102 into the light pipe 100. The second and preferred
embodiment of the light source is a lamp 108 which is a cold cathode,
reflector florescent lamp having an aperture located adjacent to the end
105 of the light pipe 100. A reflector 106 may be used with the lamp 108.
For use with monochrome displays D a cold cathode lamp is preferred to
keep power consumption at a minimum, the backlight S5 being sufficiently
efficient that the added light output is not considered necessary.
However, if a color display D is used, a hot cathode lamp is preferred
because of the need for maximum light output. Additionally, a reflector
lamp is preferred to an aperture lamp for lamps of the diameter preferably
being used in the preferred embodiment. A reflector lamp has a first
internal coating of the reflective material, which then has an aperture
developed and is finally completely internally coated with phosphor. The
aperture lamp is first coated internally with the reflective material,
then with the phosphor and finally the aperture is developed. Given the
relatively large arc of the aperture, the additional phosphor present in
the reflector lamp more than offsets the lower brightness because the
light must travel through the phosphor coating the aperture. An index
matching material 107 may optionally be provided between the lamp 108 and
the light pipe 100.
As shown the upper surface of the light pipe 100 is planar, specular and is
adjacent a low trapping and low scattering diffuser 111. The diffuser 111
preferably produces less than 10% brightness drop and is used to reduce
the effects of any moire pattern developed between the light pipe 100 and
the LCD display D because of the pitch and alignment variations between
the items. The LCD display D is located over the diffuser 111. A back
surface reflector 126 is located parallel to the back surface 112 of the
light pipe 100 to reflect light through the back surface 112 back through
the light pipe 100 and out the front surface 110. In the macroscopic view
of FIG. 5 the back surface 112 of the light pipe 100 appears to be a
straight wedge or planar surface but in the enlarged views shown in FIGS.
6 and 7 the stair stepped or faceted structure is clearly shown.
The back surface 112 consists of a series of portions 114 parallel with the
front surface 110, with a series of facets 116 leading to the next
parallel portion 114. FIG. 6 is the enlarged view showing the coupling of
the apertured lamp 108 with the light pipe 100, while FIG. 7 shows the
central portion of a dual source light pipe 100. Preferably the lamp 108
is a fluorescent type lamp with an aperture height approximating the
thickness of the light pipe 100. The light pipe 100 preferably has a
thickness of 5 mm or less at the outer edges and a thickness of 1 mm in
the center. The thickness of 1 mm is preferred because the light pipe 100
is preferably made of polymethyl methacrylate (PMMA) and so this minimum
thickness is provided for mechanical strength reasons. Other materials
which can develop and maintain the faceted structure may be used to form
the light pipe 100. The light pipe 100 is restrained to a thickness of
approximately 5 mm so that when combined with the LCD D, the reflector 126
and the diffuser 111 of the preferred embodiment, the overall unit has a
thickness of less than 1/2 of an inch, not counting the lamp 108, thus
saving a great deal of space as compared to prior art light curtain
designs. The lamp 108 can have a diameter greater than the thickness of
the light pipe 100, allowing a narrower aperture, as shown in FIGS. 5 and
6, or preferably can have a diameter approximately equal to the thickness
of the light pipe 100 as shown in FIGS. 5 and 11, with an angularly larger
aperture.
If the preferred cold cathode lamp is used as the lamp 108, the lamp 108
may run at temperatures below the optimum efficiency temperature because
of the small size of the lamp 108. Therefore it is preferable to use a
reflector 106 which is also insulating. Four alternate embodiments are
shown in FIGS. 14-17. In the embodiment of FIG. 14, a U-shaped insulator
150 is used. Inside the insulator 150 and before the light pipe 100 can be
a white reflective material 152. This material 152 can be adhesively
applied if needed, but preferably the insulator 150 is formed of a white,
reflective material. The presently preferred material is a high density
polystyrene foam, but silicone, polyethylene, polypropylene, vinyl,
neoprene or other similar materials can be used. A double sided adhesive
layer 154 is used to retain the insulator 150 to the light pipe 100. The
insulator 150 traps the heat produced by the lamp 108, thus raising the
lamp operating temperature and, as a result, its efficiency. It is
desireable that the insulator 150 and associated materials be able to
withstand 100.degree. C. for extended periods and have a moderate fire
resistance.
In the variation of FIG. 15, an expanded polystyrene block 156, or similar
material, is combined with two strips of foam tape 158 to form the
insulating reflector 106. Preferably the adhesive surface of the tape 158
includes a mylar backing for strength. In the variation of FIG. 16 foam
tape 158 is again used, but this time longitudinally with the lamp 108 to
form a U-shape. Preferably the inside of the U is covered by a reflective
tape 160, while the foam tape 158 is fixed to the light pipe 100 by a
double sided metallized mylar tape 162.
Yet another variation is shown in FIG. 17. A clear acrylic material 164
surrounds the lamp 108 and is attached to the light pipe 100 by a suitable
adhesive layer. The outer surface 166 of the acrylic material 164 is
coated with metallizing material 168 so that the outer surface 166 is a
reflector. In this manner light which is emitted from the lamp 108 at
locations other than the aperture is reflected through the acrylic
material 164 into the light pipe 100, instead of through the lamp 108 as
in FIGS. 14 to 16. While the acrylic material 164 will provide some
insulation, it may not be sufficient to raise the lamp 108 temperature as
desired and so foam insulating tape 158 may be used over the acrylic
material 164 for better insulation. In this case the entire inner surface
of the foam tape 158 may be adhesive coated as the reflective layer is
present on the acrylic material 164.
A separate injector 104 may be used to couple the light being emitted by
the lamp 108 into the light pipe 100, but preferably the end 105 of the
light pipe 100 is considered the injector. The injector 104 or end 105 is
preferably a flat surface which is polished and specular, that is
non-diffuse, and may be coated with anti-reflective coatings. A flat,
specular surface is preferred with a light pipe material having an index
of refraction greater than 1.2, which results in total internal reflection
of any injected light, which the facet structure will project out the
front surface 110.
Several other alternatives are available for the injector, such as index
matching material 107 to match the lamp 108 to the light pipe 100 to
eliminate surface reflections. The index matching material 107 is a clear
material, such as silicone oil, epoxy or polymeric material, which
contacts both the lamp 108 and the end 105. Alternatively, the injector
118 can be shaped to conform to the lamp 108 with a small air gap (FIG.
11). This curved surface of the injector 118 helps locate the lamp 108.
Additionally, a cylindrical fresnel lens can be formed on the end 105 or
separate injector 104 to help focus the light being emitted from the lamp
108. Its noted that a cylindrical fresnel lens is preferred over a true
cylindrical lens to limit leakage of the light. Alternate lenses can be
developed on the separate injector 104 or end 105 which in combination
with the facets 116 can effect the output cone of the light as it exits
the light pipe 100. Preferably the output cone is the same as the viewing
angle of the LCD D so that effectively no light is being lost which is not
needed when viewing the LCD D, thus increasing effective efficiency of the
system.
FIG. 8 shows a greatly enlarged view of a portion of one facet 116 and
several parallel portions 114 of the light pipe 100. As can be seen the
parallel back surface portions 114 are parallel with the front surface
110, both of which are specular, so that the light pipe 100 preferably
utilizes only specular reflections and does not utilize diffuse reflection
or refraction, except in minor amounts. By having primarily only specular
reflections it is possible to better control the light so that it does not
leave the light pipe 100 in undesired directions, thus allowing better
focusing and less diffusion. Thus the basic propagation media of the light
pipe 100 is that of a parallel plate light pipe and not of a wedge or
quadratic. The facet 116 preferably has an angle .alpha. of 135 degrees
from the parallel portion 114. This is the preferred angle because then
light parallel to the faces 110 and 114 is transmitted perpendicular to
the light pipe 100 when exiting the front face 110. However, the angle can
be in any range from 90 to 180 degrees depending upon the particular
output characteristics desired. The pitch P (FIG. 6) or distance between
successive facets 116 is related to and generally must be less than the
visual threshold of the eye which, while proportional to the distance the
eye is from the LCD D, has preferred values of 200 to 250 lines per inch
or greater. In one embodiment without the diffuser 111 the pitch P is
varied from 200 lines per inch at the ends of the light pipe 100 near the
lamps 108 to 1000 lines per inch at the center so that more reflections
toward the front face 110 occur at the middle of the light pipe 100 where
the light intensity has reduced. The pitch in the center is limited to
1,000 lines per inch to provide capability to practically manufacture the
light pipe 100 in large quantities, given the limitations of compression
or injection molding PMMA. If the diffuser 111 is utilized, the pitch can
go lower than 200 lines per inch because of the scattering effects of the
diffuser 111. The limit is dependent on the particular diffuser 111
utilized. Thus the use of the diffuser 111 can be considered as changing
the limit of visual threshold. In one embodiment of the present invention
the facet height H (FIG. 8) ranges from approximately I micron near the
end 105 to 10 microns near the middle, the farthest point from a lamp. In
the drawings the facet height is greatly enlarged relative to the pitch
for illustrative purposes. The preferred minimum facet height is 1 micron
to allow the light pipe 100 to be developed using conventional
manufacturing processes, while the preferred maximum facet height is 100
microns to keep overall thickness of the light pipe 100 reduced. It is
noted that increasing the facet height of a facet 116 at any given point
will increase the amount of light presented at that point, referred to as
the extraction efficiency, so that by changing the pitch P, facet height H
and facet angle .alpha. varying profiles and variations in uniformity of
the light output from the front surface 110 can be developed as needed.
While the desire is to use purely specular reflective effects in the light
pipe 100, some light will be split into transmitted and reflected
components. Even though there is total internal reflection of light
injected into the light pipe 100 by the front surface 110 and parallel
portions 114, when the light strikes a facet 116 much of the light will
exceed the critical angle and develop transmitted and reflected
components. If the light is reflected from the facet 116, it will
preferentially be transmitted through the front surface 110 to the viewer.
However, the transmitted component will pass through the back surface 112.
Thus a reflective coating 122 may be applied to the facet 116. This
reflective material 122 then redirects any light transmitted through the
facet 116. This is where the greatest amount of transmission is likely to
occur because of the relatively parallel effects as proceeding inward on
the light pipe 100.
A design trade off can be made here based on the amount of light exceeding
the critical angle being reflected back from the front surface 110,
through the back surface 112 or through the facets 116. If there is a
greater amount of this light which will be transmitted out the back
surface 112 and lost, it may be desirable to fully coat the back surface
112 as shown in FIG. 10 so that the entire back surface 112 is coated by a
reflector material 124. Because the reflector material is preferably
aluminum or other metals the efficiency of the reflector 124 is not 100%
but typically in the range of 80% to 90%, some reflective loss occurs at
each point. Thus there is some drop in efficiency at each time the light
impinges on the reflector 124, but based on the amount of high angle light
present, more light may actually be transmitted through the front surface
110, even with the reflective losses. If the lamp transmits much more
parallel light, then the coating of the parallel portions 114 with
reflective material may not be necessary.
In the embodiments shown in FIGS. 9A and 9B no reflective coatings are
actually applied to the light pipe 100 but instead a reflector plate 126A
or 126B is located adjacent the back surface 112 of the light pipe 100. In
the preferred embodiment shown in FIG. 9A, the reflector plate 126A is
planar and has a white and diffuse surface 170 facing the back surface 112
of the light pipe 100. The surface 170 is highly reflective and high
scattering to reflect the light passing through the back surface 112 back
through the light pipe 100 and out the front surface 110. The thickness of
the reflector plate 126A is as needed for mechanical strength.
In an alternate embodiment shown in FIG. 9B, the front or light pipe facing
surface 132 of the reflector plate 126B has a sawtoothed or grooved
surface, with the blaze angle .beta. of the sawtooth being in the range of
30 to 60 degrees, with the preferred angle being approximately 40 degrees.
The pitch W of the sawteeth is different from the pitch P of the light
pipe facets to to reduce the effects of moire pattern development between
the light pipe 100 and the reflector 126B. The pitches are uniform in the
preferred embodiment and are in the range of 1-10 mils for the facets and
1-10 mils for the reflector grooves, with the preferred facet pitch P
being 6 mils and the sawtooth pitch W being 4 mils. The sawtooth pitch W
can be varied if the facet pitch P varies, but a constant pitch is
considered preferable from a manufacturing viewpoint. The thickness of the
reflector plate 126B is as needed for mechanical support.
Additionally, the longitudinal axis of the sawteeth is slightly rotated
from the longitudinal axis of the facets to further reduce the effects of
moire pattern development. The sawtooth surface 132 is coated with a
reflecting material so that any impinging light is reflected back through
the light pipe 100 as shown by the ray tracings of FIG. 9. Further, the
sawteeth can have several different angles between the preferred limits to
better shape the light exiting the light pipe 100.
The majority of the light which impinges on the sawtooth surface 132 or the
diffuse surface 170 will proceed directly through the light pipe 100 and
emerge from the front face 110 because the light pipe 100 is effectively a
parallel plate because the facet area is only a very small percentage as
compared to the flat portion of the back surface 112. Thus the light which
exits the back surface 112 of the light pipe 100 is reflected back through
the light pipe 100 to exit the front surface 110 and contribute to the
emitted light with little loss.
Additionally, the actual facet profile 116 is not necessarily planar. As
shown in FIG. 12, the actual facet profile may be slightly concave 128 or
slightly convex 130. The facets 116 then form a lenticular array and can
be curved as desired to help tailor the output profile of the light cone.
Additionally, the facet 116 surface may be roughened to increase
scattering if desired.
While the design of the light pipe 100 illustrated in FIG. 5 use lamps at
both ends in a dual light source arrangement, light could be provided from
only one end in a single source configuration as shown in FIG. 13. The end
opposite the light source 102 is then the thinnest portion of the light
pipe 100' and a reflective surface 134 is provided to limit losses from
the end of the light pipe 100'. The light pipe 100' still has the planar
front surface 110, a faceted back surface 112, a reflector plate 126 and a
low loss diffuser 111 and the other variations described above are
applicable. The facet pitch and height are preferably varied as previously
described to develop greater light redirection to help compensate for the
lesser total amount of light supplied by the light source 102.
Having described the invention above, various modifications of the
techniques, procedures, material and equipment will be apparent to those
in the art. It is intended that all such variations within the scope and
spirit of the appended claims be embraced thereby.
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